Powdered metal alloy composition for wear and temperature resistance applications and method of producing same

ABSTRACT

A powder metal steel alloy composition for high wear and temperature applications is made by water atomizing a molten steel alloy composition containing C in an amount of at least 3.0 wt %; at least one carbide-forming alloy element selected from the group consisting of: Cr, V, Mo or W; an O content less than about 0.5 wt %, and the balance comprising essentially Fe apart from incidental impurities. The high carbon content reduces the solubility of oxygen in the melt and thus lowers the oxygen content to a level below which would cause the carbide-forming element(s) to oxidize during water atomization. The alloy elements are thus not tied up as oxides and are available to rapidly and readily form carbides in a subsequent sintering stage. The carbon, present in excess, is also available for diffusing into one or more other admixed powders that may be added to the prealloyed powder during sintering to control microstructure and properties of the final part.

This divisional application claims priority to U.S. Utility applicationSer. No. 12/419683, filed Apr. 7, 2009 and U.S. Provisional ApplicationSer. No. 61/043,256, filed Apr. 8, 2008, both of which are incorporatedherein by reference.

TECHNICAL FIELD

This invention relates generally to powdered metal hard prealloyed steelcompositions suitable for compacting and sintering alone or admixed withother powder metal compositions to form powdered metal articles, and tomethods of producing such hard alloy steel powders and parts madetherefrom.

BACKGROUND OF THE INVENTION

High hardness prealloyed steel powder, such as tool steel grade ofpowders, can either be used alone or admixed with other powder metalcompositions in the powder-metallurgy production of various articles ofmanufacture. Tool steels contain elements such as chromium, vanadium,molybdenum and tungsten which combine with carbon to form variouscarbides such as M₆C, MC, M₃C, M₇C₃, M₂₃C₆. These carbides are very hardand contribute to the wear resistance of tool steels.

The use of powder metal processing permits particles to be formed fromfully alloyed molten metal, such that each particle possesses the fullyalloyed chemical composition of the molten batch of metal. The powdermetal process also permits rapid solidification of the molten metal intothe small particles which eliminates macro segregation normallyassociated with ingot casting, In the case of highly alloyed steels,such as tool steel, a uniform distribution of carbides can be developedwithin each particle, making for a very hard and wear resistant powdermaterial.

It is common to create the powder through atomization. In the case oftool steels and other alloys containing high levels of chromium,vanadium and/or molybdenum which are highly prone to oxidation, gasatomization is often used, wherein a stream of the molten alloy ispoured through a nozzle into a protective chamber and impacted by a flowof high-pressure inert gas such as nitrogen which disperses the moltenmetal stream into droplets. The inert gas protects the alloying elementsfrom oxidizing during atomization and the gas-atomized powder has acharacteristic smooth, rounded shape.

Water atomization is also commonly used to produce powder metal. It issimilar to gas atomization, except that high-pressure water is used inplace of nitrogen gas as the atomizing fluid. Water can be a moreeffective quenching medium, so that the solidification rates can behigher as compared to conventional gas atomization. Water-atomizedparticles typically have a more irregular shape which can be moredesirable during subsequent compaction of the powder to achieve agreater green strength of powder metal compacts. However, in the case oftool steels and other steels containing high levels of chromium,vanadium and/or molybdenum, the use of water as the atomizing fluidwould cause the alloying elements to oxidize during atomization and tiethese alloying elements up making them unavailable for reaction withcarbon to form carbides. Consequently, if water atomization wereemployed, it may need to be followed up by a separate oxide reductionand/or annealing cycle, where the powder is heated and held at anelevated temperature for a lengthy period of time (on the order ofseveral hours or days) and in the presence of a reducing agent such aspowdered graphite, or other source of carbon or other reducing agent orby another reducing process. The carbon of the graphite would combinewith the oxygen to free up the alloying elements so that they would beavailable for carbide formation during the subsequent sintering andtempering stages following consolidation of the powder into greencompacts, It will he appreciated that the requirement for the extraannealing/reducing step and the addition of graphite powder adds costand complexity to the formation of high alloy powders via the wateratomization process.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method is provided forproducing high alloy steel powder containing at least one of molybdenum,chromium, tungsten or vanadium using water atomization but in a mannerthat protects the oxidation-prone alloying element(s) from oxidizingduring atomization so that the alloying element(s) are available to formcarbides.

According to another aspect of the invention, the carbon level in thehigh alloy steel is significantly increased above what isstoichiometrically needed to form the desired carbides. The increasedcarbon has the beneficial effect of significantly reducing thesolubility of oxygen in the molten steel, thus suppressing the oxygenlevel in the melt. By effectively reducing the oxygen level, the alloyelements are less prone to oxidization in the melt and duringatomization. Consequently, one or more of the alloying elements ofmolybdenum, chromium, tungsten and/or vanadium remain free following themelt and atomization to combine with the carbon to achieve a finelydispersed, high volume concentration of carbides in the particle matrix.Thus, the high concentration of carbon serves as both in a protectiverole by reducing the oxygen content in the melt to keep the alloyelements from oxidizing and in a property development role by latercombining with the unoxidized free alloy elements to produce a highconcentration of finely dispersed carbides in the powder duringsintering. The result is a fully alloyed powder that is inexpensivelyproduced and with an elevated hardness that is believed to be above thattypically achieved by either gas or conventional water atomizedprocesses with comparable alloy compositions having lower carbon levels.The high carbon water-atomized powder also avoids the need forsubsequent thermal processing (extended annealing and/or oxidereduction) as is necessary with low carbon levels to reduce oxygen andproduce the appropriate microstructure.

According to another aspect of the invention, the “high” amount ofcarbon included in the alloy composition is defined as an amount inexcess of the stoechiometric amount of carbon required to form thedesired type and volume percentage of carbides in the particles. Thepercentage of carbon deemed to be “high” may thus vary depending uponthe particular alloy composition.

According to another aspect of the invention, a low cost high alloysteel powder is produced by the above water atomization process. Thewater-atomized powder alloy contains at least one alloy selected fromthe group consisting of: Cr, V, Mo car W and has a C content of at least3.0 wt %.

According to another aspect of the invention, a low cost water-atomizedtool steel alloy powder is provided having a C content of at least 3 wt.%, a Cr content above 10 wt. %, a Mo content below 5 wt. % and an oxygencontent below about 0.5 wt. %, with about about 0.2 wt. % oxygen havingbeen achieved, In the as-atomized state, the carbide-forming alloys arepresent in a super saturated state due to the rapid solidification thatoccurs during water atomization. The unoxidized super saturated state ofthe alloying elements combined with the high carbon content allowscarbides to precipitate and fully develop very quickly (within minutes)during the subsequent sintering stage without the need for an extendedprior annealing cycle (hours or days)., although the powder can beannealed if desired, for example, from 1 to 48 hours at temperatures ofabout 900-1100° C., or according to other annealing cycles if desired.It is understood that annealing is not mandatory, but is optional. Ahigh volume percent of carbides can be produced (on the order of about47-52 vol %) and the carbides are uniformly dispersed and very fine(about 1 to 2 μm). The resultant high volume density carbideprecipitates provides fur a very hard powder, having a microhardness inthe range of 1000-1200 Hv₅₀.

According to a further aspect of the invention, a specific alloycomposition has been made having, in weight percent, 3.8 C, 13 Cr, 4 V,1.5 Mo and 2.5 W, with the balance being essentially Fe, The powderparticles after sintering have a volume fraction of chromium-richcarbides of about 40-45 vol % and vanadium-rich carbides of about 7 vol%. The chromium-rich carbides have a size of about 1-2 pm The particleshave a microhardness of about 1000 -1200 Hv₅₀. These properties can beessentially maintained through sintering and tempering, including ahardness above 1000 Hv₅₀, although some of the excess carbon containedin the particles above that needed to develop the carbides may diffuseout of the hard particles if admixed with another ferrous powdercomposition having a lower carbon content, This excess carbon diffusionhas the added benefit of eliminating or at least decreasing the need furadditions of carbon-rich powders (e.g., powder graphite) that issometimes added during compaction and sintering for control ofmicrostructure and property enhancement, In addition, prealloyed carbonwill reduce the tendency for graphite segregation which can occur withseparate graphite additions.

According to a further aspect of the invention, the water-atomizedpowder is mechanically ground after atomizing to break and separate outany outer oxide skin that may have formed during water atomization. Itis to be appreciated that while the outer surface of the particle maybecome oxidized even with the increased carbon content of the alloy, thealloy constituents within the particle are protected from oxidationduring the melt and atomizing. In some cases, the O content may be lowenough (such as below 0.03 wt %) where any oxide on the surface of thepowder is minimal and may be tolerated without removal, thus makinggrinding optional in some cases for at least the purpose of breaking theouter oxide layer. The mechanical grinding can be advantageously used toboth reduce the size of the particles and to reduce the effective oxygencontent of the particles by breaking off the outer oxidized layer ofmaterial, if desired, that may have formed during water atomization.

According to a further aspect of the invention, additions of sulfur,manganese, and other elements, including incidental and/or unavoidableimpurities, which do not impair the desired properties of the alloy arealso contemplated within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will becomemore apparent to those skilled in the art from the detailed descriptionand accompanying drawing which schematically illustrates the processused to produce the powder.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A process for producing high carbon, high alloy steel powder isschematically illustrated in the sole drawing FIG. 1.

A molten hatch 10 of the fully alloyed steel is prepared and fed to awater atomizer 12, where a stream of the molten metal 10 is impacted bya flow of high-pressure water which disperses and rapidly solidifies themolten metal stream into fully alloyed metal droplets or particles ofirregular shape. The outer surface of the particles may become oxidizeddue to exposure to the water and unprotected atmosphere. The atomizedpowder is passed through a dryer 14 and then onto a grinder 16 where thepowder is mechanically ground or crushed. A ball mill or othermechanical reducing device may be employed. The mechanical grinding ofthe particles fractures and separates the outer oxide skin from theparticles. The particles themselves may also fracture and thus bereduced in size. The ground particles are then separated from the oxideto yield water-atomized powder 18 and oxide particles 20. The powder 18may be further sorted for size, shape and other characteristics normallyassociated with powder metal.

The batch 10 of alloy steel is one that has a high alloy content and ahigh carbon content and a low oxygen content. The alloy content includescarbide-forming elements characteristic of those employed in tool steelgrade of steels, namely at least one of chromium, molybdenum, vanadiumor tungsten. The “high” content of carbon is defined as that in excessof the amount which is stoichiometrically needed to develop the desiredtype and volume of carbides in the particles. The “low” oxygen contentmeans oxygen levels below about 0.5 wt%.

One reason for adding the excess carbon in the melt is to protect thealloy from oxidizing during the melt and during atomization. Theincreased carbon content of the steel decreases the solubility of oxygenin the melt. Depleting the oxygen level in the melt has the benefit ofshielding the carbide-forming alloy constituents from oxidizing duringthe melt or during water atomization, and thus being free to combinewith the carbon to form the desired carbides during sintering. Anotherreason for the high level of carbon is to ensure that the matrix inwhich the carbides precipitate reside is one of essentially martensiteand/or austenite, particularly when the levels of Cr and/or V are high.

For at least cost reasons, there is a desire to increase the amount ofsome of the carbide-forming alloy elements over others. Thus, while Mois an excellent choice for forming very hard carbides with a highcarbide density, it is presently very costly as compared, to say, Cr,So, to develop a low cost tool grade quality of steel that is at leastcomparable in performance to a more costly and conventional M2 grade oftool steel, it is proposed to replace more expensive forming elementswith less expensive elements while increasing the carbon content toachieve the desired end result by way of properties and cost structure.This is done by adding to the steel alloy Cr at an amount of at least 5wt. %, reducing the Mo to less than 1.5 wt. % and increasing the amountof C to above 3 wt %. Additions of V, W can vary depending upon thedesired carbides to be formed. Table 1 below shows an example of aspecific alloy composition LA prepared in connection with the presentinvention, along with the composition of commercial grade of M2 toolsteel for comparison.

TABLE 1 Alloy compositions (in wt. %) Powder Cr V Mo W C Fe LA 13 4 1.52.5 3.8 bal. M2 4 7 5 6 0.85 bal.

Inventive powder LA was prepared according to the process describedabove and schematically illustrated in the drawing figure. It was shownto have a very high volume % of chromium-rich carbides, on the order ofabout 40-45 vol. %, and vanadium-rich carbides on the order of about 7vol. %. The chromium-rich carbides have a size of about 1-2 μm and theV-rich carbides have a size of about 1 μm. The surrounding matrix of theparticles in which the carbides were precipitated was essentiallyrnartensitic with essentially no ferrite. Austenite may be permissible.The microhardness of the LA particles was measured to be in the range ofabout 1000 -1200 Hv₅₀ in the sintered condition. The hardness wasmaintained above a 1000 Hv₅₀ after compacting, sintering and temperingwhen the LA particles were admixed as hard particles at 15 and 30 vol. %with a primary low carbon, low alloy powder composition. Some of thecarbon from the hard particles was shown to have diffused into theneighboring lower carbon content primary powder matrix material of theadmix. Controlling the sintering and tempering cycles allows one tocontrol the properties of the primary matrix, including varying amountsof ferrite, perlite, bainite and/or martensite. Additions, such as MnSand/or other compounds may be added to the admix to alter the propertiesof the admix, for example to improve machinability. The LA hardparticles remain essentially stable and their properties essentiallyuninhibited by subsequent heat treatments employed to develop theproperties of the primary matrix material.

The invention has been described in connection with presently preferredembodiments, and thus the description is exemplary rather than limitingin nature. Variations and modifications to the disclosed embodiment maybecome apparent to those skilled in the art and do come within the scopeof the invention. Accordingly, the scope of invention is not to belimited to these specific embodiments, but is defined by any ultimatelyallowed patent claims.

What is claimed is:
 1. A method of making powdered metal, comprising:preparing a molten steel alloy composition containing C, at least onecarbide-forming alloy element selected from the group consisting of Cr,V, Mo or W, and the balance comprising essentially Fe apart fromincidental impurities; water atomizing the molten alloy to yieldprealloyed powder metal particles; and during the preparation of themolten steel alloy, controlling the amount of carbon added so that thecarbon content exceeds that required to combine with the at least onecarbide forming alloy element to produce carbides during a subsequentsintering stage, and thereby defining an excess carbon constituent whichhas the effect of decreasing the solubility of oxygen in the moltensteel alloy and protecting the at least one carbide-forming alloyelement from substantially oxidizing during the water atomization. 2.The method of claim 1, wherein the carbon content is at least 3.0 wt %and having an O content less than about 0.5 wt %.
 3. The method of claim1 including selecting at least Cr as the carbide-forming alloy clementin an amount greater than 10 wt %.
 4. The method of claim 3, includingselecting the Cr content to be about 13 wt %.
 5. The method of claim 1including selecting at least Mo as the carbide-forming alloy element inan amount below 5 wt %.
 6. The method of claim 1 wherein the at leastone carbide-forming alloy element is supersaturated in thewater-atomized powder.
 7. The method of claim 1, including compactingand sintering the powder metal and causing the carbon to combine withthe at least one carbide-forming alloy element to form carbides.
 8. Themethod of claim 7, including admixing the prealloyed powder with anotherpowder and causing at least some of the carbon in the prealloyed powderto diffuse into the admixed powder during sintering.
 9. The method ofclaim 1, including mechanically grinding the prealloyed powder prior tosintering.
 10. The method of claim 1, including annealing the prealloyedpowder prior to sintering, wherein at least a fraction of the at leastone carbide-forming alloy clement is present in a supersaturated state.11. The method of claim 1, wherein the prealloyed powder is unannealedand unground before sintering.
 12. The method of claim 1, wherein Cr isat about 13 wt %, Mo is at about 1,5 and further including V at about 4wt% and W at about 2.5 wt%.
 13. A method for making a sinteredcomprising: preparing a molten steel alloy composition containing C inan amount of at least 3.0 wt%; at least one carbide-forming alloyelement selected from the group consisting of: Cr, V, Mo or W; an Ocontent less than about 0.5 wt %, and the balance comprising essentiallyFe apart from incidental impurities; water atomizing the molten steelalloy to produce prealloyed powder; compacting and sintering theprealloyed powder either alone or admixed with another powder to causethe carbon to combine with the at least one carbide-forming alloyelement to produce carbides.
 14. The method of claim 13, whereinfollowing water atomization, the at least one carbide-forming alloyelement is supersaturated.
 15. The method of claim 13, wherein theprealloyed powder is admixed with another powder and during sintering,some of the carbon diffuses from the prealloyed powder into the admixedpowder.
 16. The method of claim 13, wherein during sintering, the carbonin the prealloyed powder combines with the at least one carbide-formingalloy element to form carbides.
 17. The method of claim 14, wherein thesintered prealloyed particles have a volume fraction of chromium richcarbides of at least 40 vol %.
 18. The method of claim 15, wherein thesintered prealloyed particles have a volume fraction of chromium-richcarbides of about 45 vol %.
 19. The method of claim 15, wherein thesintered prealloyed particles have a volume fraction of vanadium-richcarbides of about 7 vol %.
 20. The method of claim 16, wherein thesintered prealloyed particles have a volume fraction of vanadium-richcarbides of about 7 vol %.
 21. The method of claim 14, wherein thesintered prealloyed particles have a volume fraction of carbides of atleast 47 vol %.
 22. The method of claim 19, wherein the carbides have asize of about 1-2 μm.
 23. The method of claim 13, wherein Cr is presentat about 13 wt %, Mo is present at about 1.5 wt %, V is present at about4 wt % and W is present at about 2.5 wt %.
 24. The method of claim 13,wherein Cr is present above 10 wt % and Mo is present below 5 wt%. 25.The method of claim 13, wherein the sintered prealloyed powder has amicrohardness of 1000-1200 Hv₅₀. 26, A sintered powder metalcomposition, comprising: at least a fraction of prealloyedwater-atomized steel powder containing C in an amount of at least 3.0 wt%; Cr in an amount of about 13 wt %; Mo in an amount of about 1.5 wt %;V in an amount of about 4 wt %; W in an amount of about 2.5 wt %; an Ocontent of less than about 0.5 wt %, and the balance comprisingessentially Fe apart from incidental impurities.
 27. The composition ofclaim 26, wherein the prealloyed water-atomized steel powder contains Cin an amount of about 3.8 wt %.
 28. The composition of claim 27including chromium-rich carbides in an amount of about 40-45 vol. %. 29.The composition of claim 28 including vanadium rich carbides in anamount of about 7 vol. %.